1. Field of the Invention
The present invention relates to the classification of abrasive grains into different categories and more specifically the separation of aluminum oxide abrasive grains into two shape categories, those grains which are appropriate for heavy duty bonded abrasives and those grains which are appropriate for both light duty bonded abrasives and coated abrasives.
2. Background of the Invention
A substantial quantity of the abrasive grains used in the world at present are produced from aluminum oxide, more specifically fused alumina which is predominantly made either from a material called corundum, which is a naturally occurring high alumina content material, or from high alumina content bauxite which is melted in an arc furnace. At present, a much larger percentage of the aluminum oxide abrasive grain produced, is derived from the arc melting of bauxite, rather than from corundum, primarily because bauxite is more readily available and less expensive.
The bauxite used is first calcined to drive off associated water of hydration, as well as moisture content. Then it is placed into an electric arc furnace along with a small percentage of metallurgical grade coke (specifically sulfur free) which serves to reduce the bauxite, thus producing brown alumina with an aluminum oxide content in a range of 94.5 to 97.5%, in comparison to the aluminum oxide content of bauxite which is 90% or less. In some cases, depending upon the desired purity of the alumina, iron turnings are also added to the melt which react with the excess oxygen and silicons that are present to form ferrosilica which gathers in the lower portion of the melt.
The arc melting furnace comprises what amounts to a very large caldron capable of holding ten tons or more of material. It is water cooled so that there is always a layer of unmelted bauxite on the inside wall of the furnace. This provides somewhat of an insulating refactory lining for the furnace. A single melting cycle is somewhat extended and can take several days. The molten aluminum oxide is then removed from the furnace and cooled into very large chunks called crude. The crude is then crushed through successive operations to form what is commonly known as grinding grit or grain. The brown alumina crude grinding grit is then subjected to alternative further crushing operations to further reduce it in size and to impose some distinction to the shape of the grains. The various shaped grains are then classified into standard grit sizes, the size classifications which include a range of particles sizes but which average out to about the indicated grit classification size.
For normal commercial purposes, abrasive grits are classified, as mentioned above, by standard grit size. The most frequently found sizes of grits range from about a 12 grit, which is relatively large, down to about a 600 grit which is very fine and is used more for polishing surfaces of materials than for removing any considerable mass of that material.
As stated above, there are two basic classifications of abrasive products which are made using abrasive grains. These are bonded abrasives, which are exemplified by what is well known as a grinding wheel, and coated abrasives, exemplified by what is well known as sand paper. Of course, in addition to these, abrasive grains, by themselves, are used for polishing and finishing purposes and may also be used for force fed abrasive purposes such as sandblasting, rotoblasting, etc.
For use in heavy duty abrasive grinding wheels, e.g., snagging wheels, after arriving at the size of the abrasive grain to be used for that grinding wheel, there is a preference, within that grit size range, for what are known as blocky grains. Blocky grains are those which usually tend to be shaped more like spheres or cubes as distinguished from flat elongated, or needlelike shapes. Blocky grains usually have an aspect ratio of about 2:1 or less. An aspect ratio is the ratio between the longest dimension spanning the two most remote opposed points on any given structure to the shortest dimension spanning the two closest opposed points on that structure. Thus, it might be said that the lower the aspect ratio, the more blocky the grain is considered to be.
The reason for desiring blocky grains in bonded abrasive products, such as heavy duty grinding wheels, is that such grinding wheels are normally subjected to a much higher amount of pressure resulting from applied force in comparison to light duty bonded abrasives or coated abrasives. Thus, the grain in a bonded abrasive product must be able to withstand shattering or crumbling under such relatively heavy force loads. Blockier grains tend to exhibit much higher strength characteristics and are not nearly as prone to shattering as those grains which are classified as sharp grains. Sharp grains, on the other hand, are those which have a relatively high aspect ratio; 3:1, 4:1 or even substantially higher aspect ratios are not uncommon in an analysis of grain shapes which are classified as sharp grains. In other words, the sharp grains are those which are the most elongated. Sharp grains are preferred for some light duty bonded applications, e.g., metal cutting tool grinding wheels, where attributes such as higher cutting rates and cooler cutting are desired. Such applications involve the imposition of substantially lower pressures and force to the grinding wheels. However, an undesirable attribute of grinding wheels containing sharp grit is a relatively high rate of wear. Sharp grit is much more frequently used in coated abrasives wherein the grit particles are glued or bonded to some sort of materials which is flexible, such as paper or cloth.
The primary reason, for selecting sharp grit for coated abrasive products, is that sharp grit particles tend to have considerably higher cutting rates at considerably lower applied forces in comparison to blocky grit. This is not to say that it is not useful, in some applications, to have some content of blocky grit among a mixture of abrasive grits used for coated abrasives. However, it should be understood that, predominantly, there would be a substantially higher content of sharp grit particles, in comparison to the content of blocky grit particles, in the selection of a grit size array which is preferred especially for coated abrasive production.
U.S. Pat. No 2,217,441 discusses the mixtures that might be appropriate in respect to the amounts of sharp grits mixed with blocky grits use in coated abrasives. This reference also describes in some detail a method of uniformly applying mixtures of the different sizes of grits within a given grit size classification to a backing material by use of electrostatic classification and separation in respect to distributing the grit size range array onto the backing material.
As mentioned previously, the crushed crude fused aluminum oxide (brown alumina) is first classified into grouped grit sizes commonly called splits. For example, a run of material from a roll crusher may be grouped into splits designated as 12/20, 24/36, 46/80 and 90/F. The 12/20 split, for example, would contain 12, 14, 16 and 20 grit size material and the 24/36 split would contain 24, 30 and 36 grit size material; the 90/F split would contain 90 and finer size grit material.
In crushing the crude aluminum oxide, from the large chunks produced by the arc furnace to the individual grit sizes, different types of crushing produce somewhat different shaped particles. For example, roll crushing tends to produce predominantly more sharp grains while impact crushing produces predominantly more intermediate to blocky shaped grains. Depending on the grit size classification of material ultimately desired, additional passes of the material through either the roll crushing process or the impact crushing process may be used to produce a greater predominance of splits of smaller (finer) grit size material. However, increasing the number of passes through either the roll crushing or impact crushing process also increases the predominance of blocky grit. When it is desired, for example, to produce blocky grit, the splits which have been size classified, i.e., that material which has been graded into a particular split size, is frequently subjected to yet another shaping operation in the form of a hammermill which tends to increase the predominance of blocky grit and produces a relatively higher percentage of blocky grit particles within the mixture, albeit a smaller grit size classification.
The roll crushing process tends to produce material which has a higher percentage of sharp grit, i.e., grit of a given size classification (split or grit size) which has a lower average bulk density and a broader range of bulk density. Roll crushing, however, is a significantly more expensive primary crushing process, and the abrasive industry, in the recent past, has increasing relied more on lower operating cost impact mills as the primary crushing means. The result is that the sharp abrasive grit available today has a higher bulk density range than that commonly available in the past, both because the sharp grit is not as sharp, on average, and because there is a somewhat higher percentage of "intermediate" grit included with the material classified as "sharp". As might be expected, this has produced an increasing degree of consternation in the customers, the manufacturers of light duty bonded products and, especially, the manufacturers of coated products.
In the past, a Sutton steel air table was used to separate or remove either distinctively blocky or distinctively sharp shaped particles from a main stream of abrasive particles and, thereby, alter the particle shape content and shape range of the grit product. The Sutton steel air table comprises an incline table which is attached to a rather strong vibration mechanism which shakes the table while forcing air through perforations in the table to slightly suspend the particles. This device is quite costly and requires a relatively high amount of energy in that the shaking operation is performed, for example, by a ten horse power or larger motor. In addition, the capacity is considered low in that it is limited to, for example, about 800 lbs./hour of 36 grit material. In addition, the Sutton-Steel air table is subject to rather frequent and high cost maintenance due to the basic conceptual design, i.e. that it is constantly shaking; the components of this equipment are considered high wear items. Of course, the cost of operation is commensurately high. There is a need for a considerably simpler type of operation, which is lower in cost, which can separate predominantly bulky abrasive grains from predominantly sharp abrasive grains.
Because it is impractical to inspect grains visually to make a determination whether or not, grain by grain, there is a predominance of sharp grains, another measure is used to classify grains as either blocky or sharp. This classification, as mentioned previously, is by bulk density or the weighted average number of grams per cubic centimeter of any given quantity of grains. For example, the production of a 36 grit grinding wheel must follow specifications; such specifications usually call for a grit which has a bulk density of between 1.85 and 1.92 g/cc. While a coated abrasive, for instance a sandpaper or a cloth abrasive, which uses the same 36 grit abrasive material will normally have a specification that calls for a bulk density of between 1.73 and 1.82 g/cc.
The blocky grit used for grinding wheels is not entirely blocky grit as mentioned before. Rather it contains 20 to 30% of sharp particles. On the other hand, the sharp grit used for coated abrasives may contain as much as 30 to 40% of blocky particles. There is a higher percentage of blocky particles in predominantly sharp grit than there are sharp particles in predominantly blocky grit. The blocky grits are produced via one or more passes through a hammermill, and only a small percentage of the sharp particles escape unbroken. Sharp grits, on the other hand, are produced by roll crushing. However, with each pass through the roll crusher, the percentage of blocky particles increases.
With extensive re-rolling, through a roll crusher, it is difficult to produce a low bulk density grit material. For example, fine (small sized) sharp grit material is readily produced as a by-product when there is a significant demand for coarser grits as only one or two passes through the rolls are required to satisfy the size range specification requirements for coarse grit. On the other hand, if a lower percentage of coarse grit or a higher percentage of fine grit are required, additional roll crushing passes are required, resulting in a progressive increase in the bulk density of the grit material with each successive pass.
To explore further the bulk density relationship in regard to grain type. A standard abrasive grit specification grain number 36 G52E was separated on a Jeffrey Table which was divided into 12 compartments to determine the shape components of the grains. The overall bulk density of the 36 G52E grit which was studied was 1.78 g/cc. After the grit was classified into the 12 different shaped components, it was grouped and various of those groupings were tested to determine the metal cut rate, or amount of metal removed, by each shape component of that grain. Table 1, following, indicates the results:
TABLE 1 ______________________________________ Weighted Jeffrey Table Weight Bulk Average Grams of Compartment Percent Density Bulk Density Cut Metal ______________________________________ 1 4.5 1.94 1.93 62 2 1.7 1.93 3 1.8 1.93 4 2.1 1.92 5 2.8 1.92 1.90 74 6 4.8 1.91 7 6.6 1.89 8 12.7 1.87 1.87 9 18.3 1.83 1.83 87 10 24.3 1.75 1.75 11 18.9 1.63 1.62 109 12 1.5 1.43 ______________________________________
It will be noted from reviewing Table 1 that those shapes from compartments 1-4 on the Jeffrey Table showed a bulk density ranging from 1.92 to 1.94 g/cc with a weighted average of 1.93 g/cc. These are the blockiest grit particles. The second grouping was removed from compartments 5, 6 and 7 having a bulk density range between 1.89 to 1.92 g/cc and a weighted average bulk density of 1.90 g/cc. Compartment 8 is the third grouping with a bulk density of 1.87 g/cc; Compartment 9 is the fourth grouping at 1.83 g/cc bulk density; Compartment 10 is the fifth grouping at a bulk density of 1.75 g/cc; and Compartments 11 and 12 are the sixth grouping with an average weighted bulk density of 1.62 g/cc and a range of bulk density of 1.43 to 1.63 g/cc.
Four of the groupings from Table 1 were mounted onto four different coated abrasive discs and tested for metal removal at a given standard amount of pressure for a standard period of time. The values shown, of grams of metal removed, are of course relative. Group four, the last group on Table 1, being the sharpest grit particles, resulted in a 76% greater amount of metal removed than the blocky grit particles of group one. Thus, it can be said that the sharper grits removed significantly more metal than the blockier grits in coated abrasive discs.
In the manufacture of coated abrasives, generally the backing material, e.g., paper or cloth, is normally coated with some type of adhesive and the abrasive grits are projected onto the surface using electrostatic energy. Those grits that do not project remain in the feed reservoir. A test was conducted to determine the relative projectability of blocky grits in relation to the projectability of sharp grits. The results of this test show that the projectability of blocky grits are relatively less than the projectability of the sharp grits. Table 2 follows and the same grade and specification of grit particles as those used in the above Table 1 test, standard grit specification number 36 G52E, were used.
TABLE 2 ______________________________________ Weighted Jeffrey Table Weight Bulk Average Project- Compartment Percent Density Bulk Density ability ______________________________________ 1 8.6 2.01 2.01 2.5 2 1.4 1.98 1.97 2.9 3 1.4 1.98 4 2.2 1.97 5 3.8 1.96 6 7.0 1.94 1.92 3.6 7 9.1 1.91 8 12.6 1.88 1.85 5.0 9 15.8 1.82 10 15.9 1.78 1.78 7.0 11 14.8 1.67 1.66 9.6 12 7.4 1.63 ______________________________________
Again, the grit particles were separated on a Jeffrey Table. Projectability is measured using an electrostatic projectability tester which comprises two 8 inch horizontal metal plates which are positioned 0.470 inches apart, placing 50 grams of a particular shape of test grits between the two plates and applying a high voltage of 8,000 volts to the plates. A resistor is also connected to both plates and the voltage generated in the circuit across the resistor is measured, it being proportional to the plate gap current in the circuit resulting from the abrasive grit particles jumping from the bottom to the top plate. The particles on the bottom plate become charged and are attracted to the top plate. The ability of those particles to jump the gap to the top plate depends on the shape of the particles. Sharp or elongated particles can become polarized and more readily attracted to the top disc; on the other hand, blocky particles show a relatively significantly less polarization and attraction.
In Table 2 it can be seen that the particle shapes from Jeffrey Table Compartment 1 had a bulk density of 2.01 g/cc. Group 2 shapes were those extracted from Jeffrey Table Compartments 2, 3, 4 and 5, having bulk densities ranging from 1.98 to 1.96 g/cc with a weighted average bulk density of 1.97 g/cc. The third group of shapes were those extracted from Jeffrey Table compartments 6 and 7 having a bulk density ranging from 1.94 to 1.91 g/cc with a weighted average bulk density of 1.92 g/cc. The group four particles were extracted from Jeffrey Table compartments 8 and 9 with a bulk density ranging from 1.88 to 1.82 g/cc with a weighted average bulk density of 1.85 g/cc. The fifth group of particles were those extracted from Jeffrey Table compartment 10 having a bulk density of 1.78 g/cc. Finally, the sixth group of particle shapes was extracted from Jeffrey Table compartments 11 and 12 having a bulk density range of 1.63 to 1.67 g/cc with a weighted average bulk density of 1.66 g/cc. The projectability of the group six particles was 3.84 times that of the group 1 particles, indicating that there is a significantly higher projectability for sharp or elongated grit particles than there is for the blocky grit particles.
There is a disadvantage in having blocky particles included in an abrasive grit used for the manufacture of coated abrasive products. During electrostatic coating, for example, as described in U.S. Pat. No. 2,217,444, the sharp particles tend to be projected while the blocky particles tend to remain in the abrasive feed material reservoir, increasing the concentration of blocky particles in that reservoir. This phenomenon results in several problems. Firstly, the coated product being produced will not have a uniform coating weight as fewer and fewer abrasive particles of any shape are projected over time, until note is taken of the problem and the power is increased. When the power is increased, the coated product then formed has a much higher percentage of blocky particles in it, usually resulting in deficient cutting performance from the coated product. The alternative is the development of an increasing residue of blocky particles, at the end of the run, which cannot be used in coated products; this, of course, increases the cost of the finished product as a substantially higher weight of abrasive grit feed material must be used to produce a given run of coated product which is made to specification.